Quantum fluids

A quantum fluid is a fluid where the mean distance between the particles is less than or comparable to the thermal de Broglie wavelength $h/{\sqrt {(2\pi mkT)}}$, where

$h$ is the Planck's constant

$m$ is the mass of the particles of the fluid

$k$ is the Boltzmann's constant and

$T$ is the temperature.

In such cases there is a strong overlap of wavefunctions of adjacent particles and hence quantum statistics are applicable. This often results in unusual observable macroscopic phenomena, such as superfluidity, superconductivity and other 'super' transport phenomena.

Reference: The extraordinary phases of liquid $^{3}$He. Nobel lecture by D.M.Lee. (http://nobelprize.org/nobel_prizes/physics/laureates/1996/lee-lecture.pdf)